This invention pertains generally to metal detectors and, more particularly, to a metal detector and method in which the effects of mineralization on target identification are eliminated.
A significant problem with metal detectors is that mineralization in the soil surrounding the target can cause the target to appear to be something other than what it is. Numerous attempts have been made to minimize this problem, but the result has remained a function of the background mineralization, with the type of target indicated depending upon the level and distribution of mineralization in the soil. The effect of the mineralization on target identification is sometimes referred to as the xe2x80x9cferrite responsexe2x80x9d.
It is in general an object of the invention to provide a new and improved metal detector and method.
Another object of the invention is to provide a metal detector and method of the above character which eliminate the effects of soil mineralization on target identification.
These and other objects are achieved in accordance with the invention by providing a metal detector and method in which two or more signals containing target information are extracted from the detector response during different phase intervals of the excitation signal and processed to eliminate ferrite response and/or response to other undesired materials. Those signals are then compared to identify targets without interference from ferrous materials or minerals which may be present.
FIG. 1 is a block diagram of a metal detector of the type to which the invention pertains.
FIG. 2 is a waveform diagram illustrating how the response of a metal detector to a given target can be broken up into a number of independent time intervals.
FIG. 3 is a diagram of one embodiment of a circuit for sampling the response of a metal detector individual time intervals.
FIG. 4 is a diagram of a circuit for gating time intervals in an excitation waveform in accordance with a control signal.
FIG. 5 is a set of waveforms illustrating the operation of the circuit of FIG. 4.
FIG. 6 is a diagram of another embodiment of a circuit for sampling the response of a metal detector individual time intervals.
FIGS. 7-8 are diagrams of circuits for sampling the outputs of a metal detector different time intervals to provide output signals which are free of ferrite response.
FIG. 9 is a block diagram of a circuit for combining the outputs of a metal detector at different time intervals to provide output signals which are free of both iron response and soil mineralization response.
FIGS. 10A-10F are circuit diagrams of a preferred embodiment of a metal detector incorporating the invention.
FIG. 11 is a timing diagram illustrating the waveforms at different points in the embodiment of FIGS. 10A-10F.
FIG. 12 is a waveform diagram illustrating the cancellation of ferrite response in the embodiment of FIGS. 10A-10F.